This invention relates to a digital halftone screen generating device for use in converting an image having continuous halftone such as photograph into a halftone image for image printing or copy on scanning image recording such as color scanner, monochromatic scanner, printer recording, facsimile recording, or the like.
In recent years, with development of electronics, in particular, of digital circuit technique, a method of electronically recording a halftone image without a contact screen has been used in scanning image technique such as the color scanner.
Such an electronic halftone image is generated as follows. An image manuscript having a continuous halftone such as photograph is converted into an image signal by a scanning image reading section such as a manuscript reading device of the color scanner or a television camera. A halftone screen generating section generates a halftone screen signal in synchronism with the scanning image reading section by referring to a periodic pattern of a halftone screen stored in a digital memory. A comparator compares the image signal with the halftone screen signal to produce a digitized binary halftone image signal. The halftone image signal is written or recorded on a paper by a scanning image writing section in synchronism with the scanning image reading section.
In color printing, in order to prevent moire in overprinting of color decomposed images, halftone image recording is carried out by changing a screen angle (halftone angle) for every color decomposed images. In addition, the number of intensity or gray levels indicated by the halftone screen may be changed in accordance with recording density in the scanning image writing section. Two methods of changing the screen angle and the number of intensity levels have been proposed as follows.
(a) One periodic pattern is stored in the digital memory. Rotation processing and reduction (or enlargement) processing are carried out on the periodic pattern to obtain a desired periodic pattern having the screen angle and the number of intensity levels (refer to Japanese Patent Application Laid Open Number 57-127362 (127,362/82)).
(b) All of necessary periodic patterns are stored in the digital memory. One of the periodic patterns is selectively used.
In the method (a), it is possible not only to save storage capacity of the digital memory but also to obtain the periodic pattern having the desired screen angle and the desired number of intensity levels. However, there is the possibility that moire occurs in the obtained halftone image signal. This is because it is difficult to obtain halftone dots with various spaces due to interpolation by the reduction (or enlargement) processing and the rotation processing.
In the method (b), it is possible to improve problem of occurrence of moire. However, it is necessary for the method (b) to preliminarily generate a number of the periodic patterns required and it results in being a burden on generation of the periodic patterns. In addition, it is necessary for the method (b) to prepare the digital memory having a large storage capacity.